Engines typically use emission control devices, such as catalytic converters, to reduce regulated emissions. However, during cold engine starting, catalysts may not be able to react these emissions since the catalyst temperature is too low. As such, the engine can be operated lean with retarded ignition timing to generate heat, and thereby raise catalyst temperature more rapidly.
The inventors herein have recognized a disadvantage with such an approach when applied with a known lean burn engine configuration. In particular, as described in U.S. Pat. No. 6,209,526, a downstream NOx absorbent is used to retain NOx emissions when operating lean, and release and reduce stored NOx when operating stoichiometric or rich. The inventors herein have recognized, however, that during cold engine starting, it takes significant time for the dowtream catalyst to reach a temperature at which stored NOx can be reacted efficiently with stoichiometric or rich exhaust gasses due to its downstream location.